The present invention relates generally to a system for precisely determining the position of a beam of incident radiation for an optical system. More particularly, the present invention relates to such a system which has particular utility in a large, orbiting telescope to accurately determine the position of a guide star relative to the telescope. The requirements of the large space telescope being considered by NASA demand that a new type of star tracker be designed. This telescope must be stabilized in its orientation to very small tolerances. The error signal for the stabilization is provided by a star tracker at the telescope focal plane that locks onto guide stars in the field of view of the telescope but outside of the data field being used for observation. The construction of the star tracker must be such that a geometrically stable relationship is maintained between the lock on point of the star tracker and the data object over periods as long as forty hours. The goal for this stability is .18 um at the focal plane. Further, the guide fields must be large enough that a high probability exists of finding suitable guide stars in the field for any data object in the sky. Because of practical limitations on the size of the guide field, the star tracker must use the guide starlight as efficiently as possible and must be noise limited only by the photoelectron noise of the sensor in order that many numerous dimmer stars may be used for guiding. The star image upon which the star tracker operates must be virtually diffraction limited in order that maximum sensitivity may be obtained. The initial pointing of the vehicle is expected to be accurate to within a diametral error of 300 urad. The star tracker must be able to find its guide star within that area and point the telescope absolutely to within about 0.12 urad.